The present invention generally relates to microelectronic assemblies, and more specifically it relates to components which facilitate connections between a microelectronic element such as a semiconductor chip and external circuit elements.
Connection components such as interposers and/or substrates are typically used in combination with microelectronic elements such as semiconductor chips to facilitate electrical interconnections between semiconductor chips and external circuit elements. The reliability of the entire circuit operation depends upon the electrical connections between the chip, the interposer and the external circuit elements.
Various attempts have been made to produce connections between the chip and the external circuit elements satisfying the above discussed requirements. For example, commonly assigned U.S. Pat. No. 5,148,265, the disclosure of which is incorporated herein by reference, discloses an advanced method for providing the connection between a semiconductor chip and external circuit elements. According to certain embodiments discussed in the ""265 patent, a semiconductor chip is connected to a corresponding substrate through a dielectric interposer. The semiconductor chip has a plurality of peripheral contacts positioned in a peripheral area of a front surface thereof. The flat, flexible interposer is formed with a plurality of connecting terminals, each of which is connected to a bonding terminal adjacent the periphery of the interposer. The flexible interposer is supported by a compliant layer. The peripheral contacts of the semiconductor chip are connected to the terminals of the interposer by bonding a multiplicity of fine leads to the chip. In one embodiment, the lead-bonding operation uses fine wires which are bonded to bonding terminals on the periphery of the interposer and to the contacts of the chip. During a wire bonding operation, when downwardly directed forces are applied to the peripheral region of the interposer containing the bonding terminals, this area of the interposer flexes downwardly. The downward movement of the interposer in the area of the bonding terminals may impede the bonding of the wires and the bonding terminals.
Commonly assigned U.S. patent application Ser. No. 08/709,127, the disclosure of which is hereby incorporated by reference, discloses a structure for compliantly interconnecting semiconductor chips and supporting substrates while substantially obviating problems associated with thermal cycling. In one preferred embodiment, the semiconductor chip package includes a sheet-like substrate having one or more gaps extending from a first surface to a second surface of the substrate and conductive terminals which are contacted from the second surface of the substrate. The substrate further has conductive leads electrically connected to and extending from each terminal and across the one or more gaps. Each lead is connected to a bond pad on the opposite side of the gap so that each lead has an expansion section within the gap which is laterally curved with respect to the plane of the substrate. In certain preferred embodiments, the expansion sections laterally curve at least twice in opposite directions and in one particular embodiment creates substantially xe2x80x9cSxe2x80x9d shaped lead portions. This structure allows the package to compensate for coefficient of thermal expansion (xe2x80x9cCTExe2x80x9d) mismatch problems by the flexing and bending of the expansion sections of the leads within the one of more gaps. The expansion sections of the leads are typically encapsulated with a compliant encapsulant to provided added support for their bending and flexing motion during thermal cycling.
Commonly assigned U.S. patent application Ser. No. 08/516,645, filed Aug. 18, 1995, the disclosure of which is incorporated herein by reference, discloses a microelectronic assembly comprising an interposer having oppositely facing first and second surfaces, a connecting terminal region and a bonding terminal region. The interposer has connecting terminals on the second surface in the connecting terminal region and has bonding terminals in the bonding terminal region. The assembly also includes a microelectronic element such as a semiconductor chip or other element having a front surface and having contacts on the front surface. The interposer overlies the front surface of the semiconductor chip with the second surface of the interposer facing upwardly away from the chip and with the first surface facing downwardly toward the chip. The connecting terminals are movable relative to the chip in vertical directions, whereas the bonding terminals are supported against such vertical movement. The interposer preferably comprises a thin, flexible layer, and a compliant layer disposed between the flexible layer and the chip for movably supporting the connecting terminal region. The assembly according to this aspect of the invention desirably also includes a reinforcing structure for reinforcing the bonding terminal region of the flexible layer against vertical movement towards the semiconductor chip. Subassemblies according to this aspect of the invention can be subjected to a bonding operation, such as a wire bonding operation, in which flexible conductors such as bonding wires are connected between the bonding terminals and the contacts on the chip. Because the bonding terminal region is reinforced, the bonding operation can be conducted efficiently. However, the finished assembly still provides the benefits associated with a compliantly mounted interposer, including testability and compensation for thermal effects during operation.
In spite of the advanced methods for providing the connection between a semiconductor chip and external circuit elements, further improvements would be desirable.
One aspect of the present invention provides a method of fabricating a microelectronic package. A method in accordance with this aspect of the invention includes the steps of providing first and second microelectronic elements having electrically conductive parts, providing an expandable structure disposed between the microelectronic elements, connecting the electrically conductive parts of the microelectronic elements together so that the microelectronic elements are electrically interconnected and expanding the expandable structure after the connecting step whereby the expandable structure increases in size so that the microelectronic elements move away from one another during the expanding step. During the connecting step, one or more flexible wires or leads are provided for electrically interconnecting the conductive parts. After the connecting step, the leads preferably have sufficient slack therein so that the microelectronic elements remain electrically interconnected during the expanding step.
In certain preferred embodiments, the expandable structure comprises a thermoplastic film or thermoplastic elastomer and a foaming agent, such as a high temperature foaming agent. The foaming agent is preferably a high temperature foaming agent so that foaming is not inadvertently triggered during processing of the package elements. The expandable structure may also comprise polypropylene with a toluene sulfonyl semicarbazide or another high temperature foaming agent, such as 5-phenyl tetrazole. As will be explained in more detail below, the expandable structure is substantially rigid during the connecting step and substantially flexible or pliant after the expanding step. During the expanding step, a sufficient amount of heat is applied to the expandable structure so that the thermoplastic film or thermoplastic elastomer will soften and the foaming agent will vaporize, causing the thermoplastic film to foam and expand. As the expandable structure expands, platens are preferably abutted against external surface regions of the microelectronic elements to maintain substantial parallelism between the microelectronic elements and to control the overall thickness of the expandable structure. In one embodiment, a first platen is abutted against the rear face of a first microelectronic element and a second platen is abutted against a central region of a second microelectronic element. After the first and second platens are in place, heat is applied to the expandable structure and the expandable structure is expanded in a controlled manner so that the first and second microelectronic elements move away from one another. After the expanding step, the one or more leads connecting the microelectronic elements have one or more curves therein so that the leads are capable of flexing and bending during thermal expansion and contraction of the microelectronic package.
In certain preferred embodiments, the first microelectronic element comprises a semiconductor chip having a front face including contacts and a rear face opposite the front face and the second microelectronic element includes a substantially flexible connection component having a first surface facing toward the front face of the semiconductor chip and a second surface facing away from the semiconductor chip, wherein the connection component has conductive terminals at the second surface thereof. In other embodiments, the second microelectronic element may comprise a substantially rigid connection component. The expandable structure is disposed between and in contact with the front face of the semiconductor chip and the first surface of the connection component. However, the expandable structure may be disposed anywhere it is desirable to place a microelectronic element in contact with an expandable structure. The expandable structure may include a plurality of complaint pads, wherein the plurality of compliant pads define channels therebetween.
After the semiconductor chip and the connection component have been electrically interconnected by bonding the conductive wires or leads to the contacts on the chip and the terminals on the connection component and after the expandable structure has been expanded, a compliant filler, such as a curable silicone elastomer encapsulant is allowed to flow within the channels between the plurality of pads. The encapsulant is then subjected to a curing process, such as using energy or a curing agent. In certain preferred embodiments, the encapsulant will constitute the bulk of the compliant interface between the connection component and the semiconductor chip because the plurality of compliant pads comprise only about 15-35% of the volume between the chip and the connection component. Preferably the plurality of compliant pads and the curable elastomer encapsulant comprise materials having substantially similar coefficients of thermal expansion. In other words, it is preferable that the plurality of compliant pads and the encapsulant comprise a substantially homogenous structure in order to avoid either thermal mismatch problems and/or the development of voids within the semiconductor chip package. However, the specific properties of the plurality of compliant pads are not as critical as the properties of the encapsulant (because the pads comprises only 15-35% of the volume) so that the array of compliant pads may comprise a more diverse assembly of materials, such as silicones and epoxies, than is possible with the encapsulant.
The present invention incorporates the realization that when bonding the ends of conductive wires or leads to the electrically conductive parts of microelectronic elements, it is critical that the electrically conductive parts of the microelectronic elements remain stationary or stable, i.e., do not move during the bonding or connecting step. Thus, it is desirable that the microelectronic elements, and specifically the bonding regions bearing electrically conductive parts, remain substantially rigid and/or stationary during the bonding operation so that effective bonds may be formed. In order to insure stable or stationary bonding surfaces, an expandable structure is provided in contact with at least one of the microelectronic elements. In a first state, the expandable structure is substantially rigid so that the electrically conductive parts of the microelectronic element remain substantially stable. While the expandable structure and the electrically conductive parts are stable, the ends of the conductive wires or leads are bonded to the conductive parts of the microelectronic element. After reliable bonds have been formed, the expandable structure is expanded by applying heat thereto, thereby causing the expandable structure to expand in size. During the expansion step, the microelectronic elements move away from one another in at least one axial direction; however, the microelectronic elements remain interconnected because the wires or leads are flexible and have sufficient slack to account for expansion and contraction of the microelectronic elements. After expansion, the expandable structure is in a second state whereby the expandable structure is substantially compliant to provide a compliant interface between the semiconductor chip and an external circuit element, such as a printed circuit board, so that the semiconductor chip remains electrically connected to the printed circuit board during thermal cycling of the chip and the external circuit element.
In one embodiment of the present invention, the expandable structure is provided between a semiconductor chip and a substantially flexible connection component; however, it is also contemplated that the connection component may be substantially rigid. The semiconductor chip includes a front face having electrical contacts and a rear face. The connection component includes a first surface in contact with the expandable structure and a second surface bearing conductive terminals. Each conductive terminal includes a central terminal for interconnecting to an external circuit, a bonding terminal at a peripheral region of the connection component, and a terminal lead integrally connected at a first end to the central terminal and a second end to the bonding terminal. After the semiconductor chip and the connection component have been juxtaposed with one another, the expandable structure is assembled between the front face of the semiconductor chip and the first surface of the connection component. Next, electrically conductive wires are bonded to the bonding terminal on the connection component and the contact on the semiconductor chip for electrically interconnecting the connection component and the semiconductor chip. The substantially rigid state of the expandable structure retains the bonding terminals in a stationary position so the bonding terminals do not move during the bonding step. After the microelectronic elements are electrically interconnected, the expandable structure is expanded by applying heat thereto during which time the microelectronic elements move away from one another. Preferably the conductive wires are flexible and have sufficient slack so that the wires are not stretched tautly during the expansion step. Moreover, the wires have sufficient slack after the expansion step to account for thermal cycling of the semiconductor chip package and the external circuit elements. After the expansion step, an encapsulant may be applied over both the expanded structure and the conductive wires to protect the microelectronic package from contamination.
In another embodiment, a tape automated bonding type lead or electroformed type lead is used. In this embodiment, the interposer is a sheet-like dielectric film having first and second surfaces. The dielectric film has conductive terminals on its second surface and flexible leads extending from the conductive terminals for electrically connecting the terminals to contacts on a semiconductor chip. The dielectric film has bond windows for accessing the flexible leads while electrically connecting the leads with the contacts. The leads are integrally formed with the conductive terminals so that the leads are only bonded to the contacts on the semiconductor chip. After the bonding step, the expandable structure is expanded as described above. Preferably, the leads have sufficient slack so that the leads are not broken or stretched tautly during the expanding step. In the next stage of the process, a low elastic dielectric encapsulant, such as a liquid silicone rubber or other curable liquid elastomer, is allowed to flow between the dielectric film and the chip and around the expandable structure and the leads while the dielectric film and chip are compressed together or held in place. A mask or coverlay may be placed over the bond windows to prevent the encapsulant from flowing through the bond windows during the encapsulation process.
In a further embodiment of a method of fabricating a microelectronic package, the leads are connected to the first or non-external surface of a connection component and the contacts on the semiconductor chip. Preferably the connection component is flexible, as described above in relation to the previous embodiment. Because the leads are connected to the first surface of the connection component, there is little room to provide vertical slack in the leads and thus lateral slack is provided in the leads. This lateral slack allows the leads to expanded in a somewhat spring-like manner as the expandable structure is expanded and/or during thermal cycling of the semiconductor chip package.
In still another embodiment of a method of fabricating a microelectronic package, the semiconductor chip is assembled in a face up, back bonded configuration. In this particular embodiment, the expandable structure is disposed between a flexible or rigid substrate and the rear face of a semiconductor chip. One or more conductive wires are wire bonded between the contacts on the chip and the bonding pads on the substrate. Each bonded wire has an expansion zone including sufficient slack to maintain an electrical interconnection between the chip and the substrate during the expanding step. In certain preferred embodiments, the expandable structure comprises a plurality of compliant pads which define channels running through adjacent pads. The adjacent pads in the array of compliant pads are preferably spaced close enough together with provide adequate support for maintaining substantial parallelism between the chip and the substrate. Preferably, the plurality of pads are compliant subsequent to the expansion step. After the expansion step, a first encapsulant, such as a curable silicone elastomer, is disposed in the channels between adjacent pads. The package may then be entirely encapsulated with a second encapsulant so that the semiconductor chip, the wires and the expandable structure are all covered by the second encapsulant. In certain embodiments, the first encapsulant and the second encapsulant may comprise the same material and/or may be deposited at the same time.
A still further embodiment of the present invention includes a fan-out semiconductor chip package wherein the leads are connected to respective chip contacts and extend outwardly beyond the periphery of the chip to terminals on a second microelectronic element, such as a dielectric element. In this particular embodiment, the first microelectronic element comprises a semiconductor chip and a package element including a heat sink. The heat sink is in the form of an open shell having a base wall, a side wall projecting upwardly from the base wall around the periphery thereof, and a projecting region extending outwardly beyond the semiconductor chip. The rear face of the semiconductor chip is bonded to the base wall of the heat sink by a thermally conductive adhesive layer. An expandable structure is provided on top of the projecting region and the dielectric element is provided over the expandable structure. The dielectric element has a top surface including electrically conductive terminals which are distributed uniformly over the entire area of the top surface. The electrically conductive leads are bonded to the chip contacts in accordance with the processes described above and the expandable structure is then expanded in a controlled manner as discussed previously. The final assembly may also be encapsulated using the encapsulation processes described above.
The foregoing and other objects and advantages of the present invention will be better understood from the following detailed description of preferred embodiments taken together with the attached drawings.